The growth and diversity of lightwave networks, such as Wavelength Division Multiplexed (WDM) networks are placing new demands on capacity management and provisioning, maintenance, and reliable and robust operation. One type of WDM network is a WDM ring network which may, for example, be used for metropolitan area network applications. The reliability problem in WDM ring networks is particularly significant since the use of closed cyclical paths makes lasing possible if the roundtrip optical gain exceeds the loop loss. The lasing affects signal channels through cross saturation and causes power variations which may reduce system power margins and cause false alarms in system channel monitoring.
A typical WDM optical ring network includes network elements with Wavelength Add/Drop (WAD) capability, whereby some optical channels are dropped and/or added while other channels are expressed, i.e., passed through. Depending on the technology used for WAD elements and the network architecture, closed loops may be formed in the WDM ring network which, with the Erbium-Doped Fiber Amplifiers (EDFA's) used to compensate the losses of network elements and fibers, can constitute ring laser cavities. Laser oscillations in the ring cavity, occurring if the round trip gain experienced by Amplified Spontaneous Emissions (ASE) exceeds the loop loss, may increase amplifier saturation and introduce additional noise, which affect the performance of optical signal transmission. Such effects have been reported by K. Bala and C. A. Brackett, J. Lightwave Tech., 14(1585), 1996 and by J. Iness, et al, J. Lightwave Tech., 14(1207), 1996.
What is needed is a technique for preventing these WDM ring networks from lasing.